This invention relates to microwave communications systems and, in particular, to a microwave communications system comprising a novel and highly effective transmitter which will allow the simultaneous transmission of a multiplicity of information channels over substantial distances.
A microwave relay provides a convenient means of getting a signal from one point to some distant point without running a cable between the two points. Microwave relays were first developed and used by the Bell System to reduce the cost of long distance telephone lines. Later, microwave links were used by the television broadcast industry to connect a studio to its transmitter station. These Studio-to-Transmitter Links (STL) were required to carry only one video channel and one audio channel from the studio to the transmitter, which might be located from one to twenty miles away. This same equipment was also used to bring network programming to the local affiliates for local broadcast.
In the early days of the CATV industry, before the use of satellite communications, these same microwave links were used to bring in distant television stations for local distribution over cable. Again, these links required only the capability of a single program channel. As the CATV industry matured, the U.S. Federal Communications Commission, recognizing the growth in usage by CATV operators, set aside a band of frequencies near 13 GHz for this service. The allocation of this band, known as the Cable Relay Service (CARS) band, has opened the way for many other applications.
One such application has been the use of a microwave relay to extend the service area of a CATV system. In order to accomplish this, the relay must be capable of carrying multiple television channels. The manufacturers of the original single-channel STL systems have responded to this demand by essentially stacking multiple transmitters in parallel in order to achieve the desired number of channels. Thus a 24-channel system would require 24 single-channel transmitters connected to a single transmitter parabolic (dish) antenna at a cost the currently exceeds $150,000. Such a cost can be accepted only if the community being connected has a very large subscriber base.
A typical microwave relay system may include a central headend system and a single transmitter with multiple dish antennas pointing toward each of several nearby communities. Within each community a single receiver aimed at the transmitter processes the signals for distribution via conventional CATV technology. A single transmitter is able to serve communities within a radius of up to twenty miles (for example) and have links in any number of directions. Thus, a single transmitter can serve a large number of communities.
By interconnecting these communities in this way, the cost of headend equipment is shared by a larger subscriber base while at the same time the cost of the trunkline system is significantly reduced. To better accomplish this cost sharing a low cost microwave relay system must be used.
By way of example, microwave relays that operate in the 12.7 to 13.2 GHz band will be described. There are two classes of microwave relay systems which can operate in this band: Amplitude Modulated (AM), and Frequency Modulated (FM). In both of these systems, a transmitter system can be shared by several links. By appropriately connecting the transmitter to dish antennas pointed in different directions, several areas can be served by a single transmitter.
Frequency modulated links provide the most reliable low noise transmission with the lowest transmitted power. Because of its reliability and noise performance, an FM link is often used for STL systems. FM is also used for mobile systems in order to reduce the required output power.
There are, however, several drawbacks to an FM system. Of primary concern is the high cost of both the transmitter and the receiver. Second, the effective bandwidth required of an FM signal for a microwave relay is four times that of a vestigial sideband AM signal. This means that, for a given bandwidth, only one-fourth as many stations can be carried. Finally, an FM system requires separate transmission and reception equipment for each television channel to be carried.
Amplitude modulated links can provide an economical method of relaying multiple television stations. With an AM transmitter all channels can be transmitted and received with a single transmitter and receiver. However, conventional amplitude modulated links do not take full advantage of the multiple channel capability of an AM system. By properly selecting the components of the multiple channel transmitter as shown below, a significant cost reduction can be achieved as compared to single channel transmitters.